Get Auctioneer and learn how to use it. Make sure you run regular scans of your AH so it's showing correct prices. Developing a good personal shopping list on auction mods makes a big difference. Sterling's post here: http://www.mmo-champion.com/threads/...ight=sterling1 is a good guide, it's a bit out of date now in some respects but in terms of teaching how to do things it's great.

Be prepared to log in fairly regularly to check auctions (unless is something where you're selling the only one).

Make sure you know your server economy well, and make sure you keep up with wow news. You need to be able to spot a bargain relatively quickly and you need to know what things are priced as they are, and you need to know when to buy, hold or sell, which changes depending on what's happening in wow generally and what's going on on your server specifically (e.g. buying cheap gems pre-patch and then holding them to sell after when everyone has new gear). Thorough knowledge of professions is also helpful for this, as it helps to know what people are going to be crafting and buying in the future so you can stockpile cheap and sell later, and also selling leveling mats is a nice way to profit.

I've made a lot more of my income from flipping this xpac than I have in the past, however to be good at it it does take good market knowledge (unless you get stupidly lucky - but even then you need to be scanning for the right thing at the right time). I'm not sure how much gold you have to start with, but if it's under 100k I'd suggest starting small, and selling lots of smaller things rather than say going right for mounts and similar items, they can take longer to sell and you don't want all your gold tied up in them (unless there's a real bargain you're positive you'll sell of course).

Reselling is one of the things that sadden me because the players who profit from that do are not actually doing anything. It makes the game less enjoyable overall for the majority of players, in my opinion.

OT tip for reselling: Get tons of Gold. Many players have a limited amount of Gold so if you get very rich, you can do more reselling. In a recent Example I saw a good Deal for Item(s) that cost ~~600 000Gold and resold for ~~624 000Gold throughout the Months. 99.9% of the players couldn't have done it even if they wanted to because they have no 600k available to them.

Get Auctioneer and learn how to use it. Make sure you run regular scans of your AH so it's showing correct prices. Developing a good personal shopping list on auction mods makes a big difference. Sterling's post here: http://www.mmo-champion.com/threads/...ight=sterling1 is a good guide, it's a bit out of date now in some respects but in terms of teaching how to do things it's great.

Sterling(OP of post you linked) is the founder of Stormspire.net, by far the best gold making site related to wow.

Auctioneer is a bit outdated, Trade Skill Master is the preferred addon used now.

Keep in mind reselling can be hard on highly populated servers, glyphs I resell have been undercut within 5 seconds multiple times for me, seriously, I've put them on, waited 5 secs then scanned undercuts and boom, everything is undercut, I don't care with glyphs because they have a ridiculously low listing price, but it can sometimes take days and days of listing the same thing before it actually sells because 10 other people are doing the same thing.

By hard I actually mean time consuming, very, time consuming, to get it right.

I also prefer Auctionator over Auctioneer, it's less powerful, but far easier to use and lightweight for general selling, the latter is more powerful for pure reselling.